Please note that I am only referencing UTY to highlight my train of thought. I am NOT trying to suggest any connections between UTY and UTDR.
I try not to draw too heavily from AU's, but I noticed a pattern in UTY that made me realize something about UTDR. So, in UTY, almost all of the bosses become op simply to defend their own pride:
Decibat fights us to shut us up so he can go back to sleep. We don't even meet him in Genocide.
Dalv has some hidden power that we never see- he says at the end of a Pacifist that he "held back," which would make us think he would be difficult in Genocide, but he's not. He just assumes he's imagining us and the whole battle plays out exactly the same as in Pacifist.
El Bailador isn't even trying to fight, he just wants to dance, and we get hurt in the process. He doesn't appear in Genocide; presumably, he fled with the other monsters.
The Feisty Four only attack in Pacifist/Neutral because they're mad at Star and taking it out on you. We never even see them in Geno.
Starlo is difficult in Pacifist because he's defending his wounded ego, he's mad at himself but taking it out on you, but we don't even get a real battle in Genocide.
Axis is always following the instructions programmed into him by a monster with a deep-seated hatred of humans.
Ceroba is ridiculously hard in Pacifist because she loathes herself and is taking it out on you, but doesn't even get a proper second phase in Geno.
In all of these cases, the only one they are protecting in Pacifist is themselves (except Axis, who is strictly following programming). When it comes down to facing a Genocidal murderer, they just can't muster enough fighting spirit to do much of anything. Heck, Ceroba is only difficult in Geno because of revenge and pure rage. She says she's protecting monsters from us, but the only real reason she's fighting is because she's pissed at us for killing Starlo.
And... we actually see a similar pattern in UTDR:
Toriel fights us under the pretense of "protecting" us (which never made much sense to me, tbh), and she can muster almost no fighting spirit. It's very difficult to force her to kill you. And we one-hit her in Geno, she does no damage, she has no defense.
Maddie is semi-difficult in Pacifist/Neutral because they claim to be taking revenge for their cousin, but she can't even remember their name, and she flees as soon as Blookie starts crying on her. In Geno, Maddie stops caring about their cousin as soon as they merge with their dummy and become Gladdie. We don't even get a proper Maddie fight in Geno.
Muffet is semi-difficult even after a couple of playthrus, and what's her reasoning? Partly to save the spiders, but her motivation is mostly money- she was paid to kill us. She doesn't give a fig about other monsters at all, just spiders. We don't even encounter Muffet in Geno.
Mettaton. What's his reason for fighting us in Pacifist/Neutral? Purely selfish: he wants to be the star, not just in the Underground, but on the surface. Killing us and giving our SOUL to Asgore is just his way of accomplishing that. He doesn't give up until: a) we blow his arms and legs off so that he can't be much of a star at all anymore; and b) he gets a call from Blookie. Remembering what actually matters in life is what makes him end the fight. And in Geno, MTT becomes a literal one-hit wonder. When he has to fight to try to SAVE the world, he just isn't cut out for the job.
Asgore. Yeah, he puts up a fight in Neutral. He doesn't want to, tho. He's tired of fighting. He's tired of killing. He doesn't really want to go to war against humanity on the surface. So we can beat him. And in Geno? Where tf was he when his entire kingdom was being decimated??? We get to the throne room and he just offers us tea!
Flowey/Asriel, doesn't matter what name he goes by, every time we fight him, he's trying to defend his position as a wannabe god and keep playing with Chara forever. He fights because he has nothing left in the world to live for. He fights because that's the only thing he can still feel, the rage and the joy of killing. As soon as we remove that feeling from Asriel, he becomes a child's ghost, and then back to a flower, but a helpless and innocent one that can't fight at all anymore.
Lancer's first battle is the only time he poses any kind of a threat, because he's fighting for the right to become "son of the month." (And even I haven't ever died to him.) He's really not a threat in the fight against him and Susie, and he starts missing in the battle between him and Susie. He was never really out to hurt us at all.
Clover isn't too difficult because she's basically just fighting with herself and we got in the middle of it.
K. Round is never a threat, either, because they never wanted to fight, they were forced into it.
Susie (when we fight her alongside Lancer) is about as difficult as one would expect by the time the fight happens. She's fighting to maintain her persona as a bully and a bad person. In all actuality, tho... she's not as tough as we would expect from someone who keeps threatening to bite people's faces off.
Jevil is clinically insane, fighting and killing for the sheer fun of it, and is crazy hard.
Spade King is difficult, and his motivation is purely selfish. He doesn't care about his people- I mean, he wanted to throw his own son off a 5-story roof! He only cares about the Knight that put him into power, about maintaining that power status, and about revenge against the Lightners that sealed him away and never returned. His difficulty in battle is fueled by rage, revenge, and a lust for power.
Sweet Cap'n Cakes claim to be on a crusade, but they really only seem to be interested in having fun and making music without restrictions. Their battle is only difficult because all three of them attack at pretty much the same time and cannot be killed. Getting all of them to dance at the same time ends the battle. Their shop in the city disappears completely in a Snowgrave.
Rouxls is... um, can we even call that a battle? You know what, let's just not even try to analyze him right now....
Spamton. His only reason for fighting, in either Normal or Snowgrave, is to become a [[BIG SHOT]]. And we know how difficult Spammy is. In reality, he's no harder in Snowgrave than in a Normal Route. It just seems harder because there's only one hero character to fight him with.
There are only 3 characters between UTY and UTDR that are more difficult in a No Mercy route than in a Normal route:
Martlet is the last boss that is highly unlikely to kill you at all in Pacifist/Neutral. She turns into sans of UTY in a Geno- and that's her second battle. She's pretty OP when we encounter her in Snowdin, too.
Undyne is difficult enough in Pacifist/Neutral, but we have the option to flee. Her reasoning is that, since she is the Captain of the Royal Guard and Monsterkind's Hero, it is her duty to take our SOUL. But in Genocide, she changes completely. Her Determination, which causes her to melt in a Neutral, suddenly gives her the power to completely transform into a god-like character. And why? Because she is the only thing that stands between us and the world. Because she can see that we aren't just a human- we're some kind of demon that will destroy everything. She's not just fighting for monsters, but for humans, as well.
sans fights us for the same reason, albeit far too late to save the monsters. But he realizes that we will tear down the rest of the world if we keep going, and something inside him decides that he can't allow that. We actually don't know why he suddenly decides to care so much, but he does. He uses every ability he has, from karma, to Gaster blasters, to bone attacks, to teleportation, to even sleeping, just to make us quit. Because he knows we'll kill Asgore. And then there will be nothing to keep us from destroying the rest of the world. We NEVER fight sans in a Pacifist/Neutral, only in Geno. So this weird little guy with only 1 HP left to his name stands up to us, knowing full well that one day we will kill him, but he is literally the ONLY thing left to stop us from destroying the entire world. He may be too late, but when he finally steps up, it's for the right reason. There is no more selfishness in his battle than there is in Undyne's. Both sans and Undyne have no interest in personal gain or glory, but in saving the world.
And then there are 3 anomalies, all of them in UTDR:
Berdly is, to my knowledge, the only character to be no more and no less difficult in Snowgrave than in Normal. (Well, maybe a little easier in Snowgrave, but that's just because of Noelle's spells, not because Berdly himself changed.) He puts up the best fight that he possibly can, no matter whether he is protecting himself, or trying to save Noelle. This, to me, suggests 2 possibilities: a) he cares just as much about Noelle as he does about himself; or b) he really was fighting for Noelle in both routes. I don't think the second option is entirely true, since in a Normal fight, he does "hit [Noelle] in the face with a tornado," and talks more about himself and how superior he is to everyone else than he does about Noelle. But even so, I think she always was a big part of his reason for fighting.
Queen is the only character to have a difficult battle (with multiple phases, no less) in a Normal Route, and no battle at all in Genocide, who is not doing it for purely selfish reasons. Granted, it seems likely that she would want to preserve her comfortable, regal lifestyle, but not only does she never cite this as a reason for fighting, she doesn't even balk at having to leave her mansion and move into Ralsei's castle. The only reason she ever gives for fighting is trying to help the Lightners (the exact opposite of her ex-husband).
I say this all the time: Papyrus CANNOT kill you. Not without the player seriously breaking the game. And as I understand, at Undertale's initial release, he could kill you, accidentally, if certain conditions were met; but this happened very rarely and has since been entirely patched out. So. For all intents and purposes, he CANNOT kill you. So the fact that there is no real Papyrus battle in Genocide is kind of a moot point, since he was never going to kill you, anyway. And, as many people have pointed out, Papyrus should be able to kill you, probably even one-hit you no matter your LV, in a Pacifist/Neutral route. But he breaks the game to make sure that doesn't ever happen, whether by lowering his own stats, moving the final large bone in his "really cool regular attack," etc.
Anyway. I said all this just to make some points about the characters: that most of them pretty well parallel real life. Truth is, most people don't give their all for the right reasons. Most people do only fight to protect themselves and their egos, and not to protect others.
